Congress returned from its election break Tuesday to grapple with the shocking resignation of former CIA Director David Petraeus in a sex scandal that widened to possibly taint the Marine general who commands U.S.-led forces in Afghanistan.

WASHINGTON -- Congress returned from its election break Tuesday to grapple with the shocking resignation of former CIA Director David Petraeus in a sex scandal that widened to possibly taint the Marine general who commands U.S.-led forces in Afghanistan.

The unexpected turn of events prompted President Barack Obama to put on hold his nomination of Marine Gen. John Allen to be the top NATO commander in Europe, pending the outcome of a Pentagon investigation into "inappropriate" emails that U.S. defense officials said Allen had sent to a central figure in the scandal, which was ignited by Petraeus' admission to an extramarital affair.

The White House said that Obama retained his faith in Allen, who is in the midst of developing a plan for the gradual withdrawal of U.S.-led international forces from Afghanistan by the end of 2014.

"The president remains focused on fully supporting our extraordinary troops and coalition partners in Afghanistan who General Allen continues to lead as he has done so ably for over a year," said White House Press Secretary Jay Carney, adding that the president "believes he's doing and has done an excel

lent job."

Some lawmakers, however, expressed dismay at reports that Allen was able to exchange more than 20,000 documents and emails with Tampa socialite Jill Kelley, even as he oversaw the longest war in the nation's history.

"It's hard to imagine you can manage an agency or run a theater of war while sending or getting so many emails, if that's what actually occurred," Sen. Richard Durbin of Illinois, the Senate's No. 2 Democrat, told McClatchy Newspapers.

Senate Armed Services Committee Chairman Carl Levin, D-Mich., who agreed to Obama's request to delay Allen's nomination hearing, which had been set for Thursday, also said he was "kind of surprised by the number of emails, but until the ... investigation concludes, I don't want to prejudge it."

The committee, however, will proceed with the confirmation hearing of Marine Gen. Joseph F. Dunford to take over from Allen as commander in Afghanistan. The handover is not expected before March.

In a related development, Senate Intelligence Committee Chairwoman Dianne Feinstein, D-Calif., said that she intended to summon Petraeus back into the spotlight to answer questions about the Sept. 11 attack on the U.S. consulate in the eastern Libyan city of Benghazi. It resulted in the deaths of the U.S. ambassador to Libya, two CIA contract security guards and a State Department staffer. The committee hasn't decided when he would be called, she told McClatchy Newspapers.

Carney said that it was "up to Congress to make decisions about who is called to testify."

Feinstein and other lawmakers have expressed concern that they weren't informed about the FBI investigation that uncovered Petraeus' affair with his biographer, Paula Broadwell, late this summer until after it became public Friday. The CIA released a Petraeus resignation statement to the agency's workforce in which the storied, retired four-star Army general admitted to committing adultery.

"Whatever the affair's effect had on national security is worth knowing about," said Sen. Lindsey Graham, R-S.C. "Why did the FBI investigate to begin with? And did this have an effect on national security?"

Allen's connection to the scandal became known on Tuesday after Defense Secretary Leon Panetta announced while flying to Asia that he'd asked the Pentagon inspector general's office to investigate the emails between the four-star Marine general and Kelley.

Kelley had been an honorary ambassador to the U.S. Central Command in Tampa, where Allen served as second in command before his June 2011 promotion to oversee the war in Afghanistan. Army Lt. Col. T.J. Taylor, a CENTCOM spokesman, said that Kelley had no official position at the command.

"She is a volunteer and a private citizen, not an employee," he said. "Because of this, and because there is an ongoing investigation, we have no additional information to provide."

The contents of those emails were not revealed, although a Pentagon official briefed on them said they contained possibly flirtatious language, such terms as "sweetheart," which he noted could also have been a sign of a casual manner of address. The official requested anonymity because he was not authorized to discuss the matter publicly.

A second Pentagon official, who requested anonymity for the same reason, told reporters that the emails weren't "security related. They're not, I don't know, information about acquisition or contracting. They're not hate or threatening."

The FBI investigation that uncovered Petraeus' affair with Broadwell began when Kelley sought help from an FBI agent after receiving what she described as anonymous and threatening emails warning her to stay away from Petraeus.

The probe led to Broadwell as the sender. She is an Army reservist, a West Point graduate and researcher who met Petraeus when he made an appearance at Harvard University.

For his part, Allen has maintained that he had done nothing inappropriate, the Pentagon officials said.

The extent of the friendships maintained with Kelley by Allen and Petraeus was highlighted by the generals' intervention in a child custody battle involving Kelley's twin sister, Natalie Khawam.

In their letters, which were first reported by the New York Post, Allen and Petraeus urged the judge overseeing the case to grant Khawam, who lost custody of her child to her ex-husband, more opportunities to spend time with her child.

"We have on many occasions observed Natalie and her son ... including when we hosted them and the Kelley family for Christmas dinner this past year. In each case, we have seen a very loving relationship," Petraeus wrote in a Sept. 20, 2012, letter. "Natalie clearly dotes on her son and goes to great lengths -- and great expense -- to spend quality time with him."

Petraeus and his wife, Holly, became friends with Kelley and her husband while serving as CENTCOM commander from October 2008 and June 2010; he had known Khawam for three years. He had served as CIA director for more than a year when he sent the letter. But he chose to write it on Army stationary embossed with a four-star flag, symbolizing his four-star rank at the time he retired from the military in July 2011. He also signed the letter as "General, U.S. Army (retired)."

In his letter, Allen said that he and his wife, Kathy, became friends with Khawam while he served as deputy CENTCOM commander from July 2008 to June 2001. He and his wife, he wrote, had seen Natalie and her son on "multiple occasions" during official CENTCOM functions.

In the halls and corridors of the Pentagon, there was little on-the-record talk of the scandal, though in private moments the situation was described time and again as "sad," "unfortunate" and even "pathetic."

Around the capital, there was speculation that, regardless of the results of the Allen investigation, he might not rebound quickly from this latest turn in the scandal.

Carney said the White House learned about the Allen emails on Friday after the Justice Department notified the White House counsel that there "may be an issue" in relation to his nomination. Obama had not spoken directly to Allen, Carney said, and he was uncertain whether Allen had talked with Panetta.

Carney said that Obama would not make any "grand pronouncements or decisions" based on the events surrounding Petraeus and Allen. But he said that the president was "certainly surprised" when he was told about Petraeus on Thursday, but that he "has confidence in the acting director of the CIA, and he has confidence in the military to carry out the various missions that he has them to carry out."

Carney also said the president "reiterates his belief that the Senate should act swiftly to confirm General Dunford as Allen's successor in Afghanistan."